avfilter/avfiltergraph: assert that the heap_bubble index is valid

This might help coverity
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avfiltergraph.c

@@ -1280,6 +1280,8 @@ static void heap_bubble_up(AVFilterGraph *graph,
 {
     AVFilterLink **links = graph->sink_links;
 
+    av_assert0(index >= 0);
+
     while (index) {
         int parent = (index - 1) >> 1;
         if (links[parent]->current_pts >= link->current_pts)
@@ -1297,6 +1299,8 @@ static void heap_bubble_down(AVFilterGraph *graph,
 {
     AVFilterLink **links = graph->sink_links;
 
+    av_assert0(index >= 0);
+
     while (1) {
         int child = 2 * index + 1;
         if (child >= graph->sink_links_count)